In an engine (internal combustion engine) mounted in a vehicle or the like, water jackets are each formed in a cylinder head and a cylinder block as coolant paths, and a coolant is circulated in the respective water jackets of the cylinder head and the cylinder block by a water pump. As such an engine cooling device, known is a device in which an exhaust heat recovery unit is disposed in a coolant path (bypass path) that bypasses the respective water jackets of the cylinder head and the cylinder block, and during engine warm-up, no coolant is circulated in the respective water jackets of the cylinder head and the cylinder block, and the coolant is circulated only in the bypass path (for example, see Patent Literature 1). The exhaust heat recovery unit is provided in, for example, the exhaust system of the engine, and is configured so as to be capable of carrying out heat exchange between the exhaust gas flowing in the exhaust path and the coolant flowing in the coolant path.
Also, air-fuel ratio feedback control is carried out in the engine based on the output of an exhaust gas sensor (for example, A/F sensor, O2 sensor, or the like) disposed in the exhaust path (for example, see Patent Literature 2). In order to stably carry out the air-fuel ratio feedback control, the exhaust gas sensor needs to be in an activated state by being sufficiently heated by the heat of the exhaust gas, and specifically, the temperature of the exhaust gas sensor (the temperature of a sensor element) needs to be equal to or higher than a predetermined activation temperature (for example, see Patent Literature 2 and 3). Patent Literature 3 states that when the temperature of the exhaust path is low, the sensor element of the exhaust gas sensor is heated by a heating means.